Place all the ingredients for the sauce, except the yoghurt in a mixing bowl and grind up with a hand blender or food processor. Once it resembles a smooth paste, mix in the yoghurt to form a light yellow sauce. You can either marinade the chicken in the sauce in the fridge for a while, or cook it straight away with the onions. Fry the onions brielfy in a little oil before placing the chicken and sauce in a large frying pan. Cook for about 20 minutes, adding some water to make it more saucy.